A typical cellular communications network is geographically divided into a number of cells and/or sectors, which are usually contiguous and which together define the coverage area of the network. Each cell is served by a base station, which includes a fixed/stationary transceiver and an antenna for wireless communications with a set of distributed mobile stations (e.g., mobile phones) that provide service to the network's users. The base stations are in turn connected to one or more mobile switching centers, each of which serves a particular number of base stations depending on network capacity and configuration. The mobile switching centers act as the interface between the wireless/radio end of the cellular network and a public switched telephone network or other network(s), including performing the signaling functions necessary to establish calls or other data transfer to and from the mobile stations.
In a cellular network, transmissions between the base and mobile stations may be carried out according to a number of possible protocols. In networks that utilize CDMA (code division multiple access) spread-spectrum communications, the mobile stations all transmit across the same frequency bandwidth, with the signals from each mobile station being differentiated from the others according to an encoding scheme. Transmissions from the base station to the mobile stations are across a frequency bandwidth known as a forward link, and communications between the mobile stations and base station are across a frequency bandwidth known as a reverse link. The forward link may include one or more logical channels for transmitting control signals and data/voice traffic. It may also include a pilot channel for transmitting a pilot signal, which is a signal whose characteristics are known to the mobile station receivers, for synchronization, handoff, and other purposes.
One type of CDMA implementation utilizes a slotted transmission scheme with a time-multiplexed pilot signal. In such a system, a single base station antenna is used for transmissions between the base station and mobile stations, with information on the forward link being transmitted in a time-slotted fashion, i.e., transmissions over the forward link are divided into time slots. In particular, each time slot is divided into portions over which a pilot signal (i.e., pilot channel), control information, and user data are transmitted in non-overlapping time zones, respectively. The time structure of the slot is known and synchronized between the base station transmitter and the mobile station receiver. Again, the pilot signal is a signal whose characteristics are known to the mobile receiver, and on which signal quality and channel estimation measurements may be made. Also, this pilot signal structure, where a portion of the time slot is allocated to pilot transmission, is referred to as a time-multiplexed pilot channel.
Signal quality measurements are made by the mobile station receiver and reported back to the base station in the form of channel quality indicator (“CQI”) feedback. The base station may then utilize the CQI to determine the mode of transmissions to a particular mobile, e.g., the data rate to use to transmit to a particular mobile.
To such a system, one or more spatially separated transmit antennas may be added to the base station, in addition to the existing antenna. These antennas are used to transmit additional pilot signals, which are referred to as “secondary” pilot signals to differentiate them from the primary pilot signal transmitted inside the time slot by the primary antenna. The secondary pilot signals are differently configured from the primary pilot signal, allowing the mobile stations to distinguish between the two. The secondary pilot signals may be transmitted one per transmit antenna, or one per transmit beam. A transmit beam refers to a signal formed by the composition of coordinated transmissions from several transmit antennas. Accordingly, as used herein, the “transmit antenna” includes a “transmitted beam.”
The secondary pilot signals can be either a broadcast pilot, i.e., targeted for and intended for all or a subset of the mobiles stations in a served area (e.g., sector or cell), or a “user specific” pilot intended for one specific mobile station.
Multiple antennas and secondary pilot signals are used to enhance system performance. In particular, due to several well-understood physical phenomena that impact the propagation of radio signals, because of the physical separation between the transmitting antennas, identical signals transmitted from two antennas at the same time lead to received signals of unequal quality at the mobile receiver. This phenomenon can be exploited, and there are many multiple antenna transmission schemes that offer superior performance. Examples include selection transmit diversity (e.g., transmitting to mobile stations from only one or a subset of antennas or beams), transmit diversity (e.g., transmitting a space-time coded signal from multiple transmit antennas), closed-loop transmit diversity (e.g., one or more beam-forming signals from multiple antennas to achieve increased signal gain at the receiver), MIMO-Code reuse (e.g., transmitting multiple data streams simultaneously to increase data rates), and fixed beam with beam switching. For enabling these multi-antenna transmission schemes, the base station needs to have a knowledge of the perceived received channel conditions across the transmit antennas. Since the primary and secondary pilot signals are transmitted from different antennas, knowledge of the channel quality measurements on these channels would imply a knowledge of channel conditions across the transmit antennas. Hence, for enabling these multiple antenna transmission schemes, the mobile stations need to make separate measurements on the primary and secondary pilot channels/signals, which are used for signal reception and/or to report required metrics back to the base station.
Because multiple antennas (along with associated secondary pilot signals, etc.) offer the opportunity for improving system performance, it may be advantageous in certain situations to outfit existing single-antenna systems with one or more additional transmit antennas. However, this raises issues of backwards compatibility, since existing mobile stations may not function properly when presented with signals from multiple antennas, e.g., multiple pilot signals.